Means for and metho of equalizing attenuation



yMay 1'2, vv'1925.

' H. A. AFFEL MEANS FOR AND METHOD 0F EQUALIZING ATTENUATION OriginalFiled June 2, 1921 n TSQNSQ MHQHWWHH..

IMWMH HHEWMHM Patented May 12, 1925.

UNiTED s'rii'rlassfA PATE- 1,537,106 A orinar-1.".

HERMAN A. Armer., or BnooxLYN, NEW Yoan, AssreNon rro AmnnrcAN TELEPHONEAND TELEGRAPH COMPANY, `A Conroe-Arron or NEW Yonx.

l MEANS FOR AND METHOD OF EQUALIZING ATTENUATION.

Application led June 2,' 1921, Serial No. 474,402. Renewed August 6,1924.

Ta all whom it may concer/n.:

Beit known that I, HrlinnANt A. AFFEL, residing atBrooklyn, in thecounty of Kings and State of New York, have invented certainImprovements in Means for and Meth? ods of Equalizing Attenuation, ofwhich the following isa specification..

This vinvention relates to the transmission of alternating currents forsignalling or other purposes, and more particularly to the equalizationofthe attenuation of the currents transmitted over a circuit atdifferent frequencies.

It is a well known fact that when variable currents such as telephone ortelegraphfcur.-

rents are transmitted over a long conductor, distortion' occurs owing tothe difference in the attenuation of the .different frequencies involvedin the transmission, the higher frequencies being subjected to greaterattenuation than the lower frequencies. This is particularly undesirablein the case of telephone transmission, as good quality of speechtransmission requires that the wave form of the signal initiated at oneend of the line must be maintained at the receiving end, and when thedistortion becomes serious, the transmission tends to becomeunintelligible. In accordance with the present invention (it is propesedto overcome this diliiculty by tras' mitting the telephone or othersignalling band in its normal condition over one half of the entirelength of a transmission system, and transmitting the band inverted overthe other half of the system. In `other words, overv one half of thecircuit the voice band is transmitted in such a manner that the-frequency relations of the band are normal, while overl the other halfof the circuit the fre uency relations are inverted so thatxthe higherfrequencies of the voice band become low frequencies, and the lower freuencies become high frequencies. The hal 'of the total system over whichthe voice currentsl are transmitted at their normal frequencies givesthe usual rising frequency-attenuation curve; thesecond half of thesystem has'a similar frequency-attenuation characteristic but thefrequency band, being reversed, in effectreverses the attenuation curveitself. For example, if a frequency of 2,000 cycles in the normalportion lof the system translated-to 200 cycles in the inverted portion,the total attenuation suffered by the 2,000

by a 200 cycle component,`which in the inverted` half of the systemappears as 2,000

cycles.

In order to invert the band when desired and to restore it to normal forreceiving purposes, advantage is taken of the principle of -modulationwhereby as a`result of modulating a carrier frequency by a transmissionband, two side bands will result on either side of the carrier, eachside band having the same 7width as the transmission band, and one' sideband having the same frequency relations as the original bands, but theother side band -l l having the frequency relations reversed. If thecarrier .be so chosen as to correspond to the upper frequency of thenormal voice band, the .lower side band resulting from modulation' willbe reversed with respect to the normal voice band, but will occupy the'same frequency range.

The invention "may now be more fully understood by referring to thefollowing detailed description when read in connection with the'accompanying drawing, Figures 1, 2 and 3 of which represent threeschematic embodiments of the invention.

Referring to Figure l, L'1 and L4 designate telephone4 circuits,incoming to stations A and' B, to which voice currents or othersignalling currents'may be applied for transmission between stations Aand B, said stations being interconnected by line sections L2 .and L3,over which it is desired to equalize the attenuation. In order toequalize the attenuation the circuit is so arranged that in transmittingfrom station A to station B, the frequency relations of the voice bandwill be inverted in transmitting over the line secfor transmission fromstation A over the` line section L2, the line section L1 is asso-lciated with the line section L2 at thestation A through a circuitarrangement having the general form of a 22 repeater circuit, comprising channels E, and W1, the channei E, being used fortransmissionfrom west to east,

.cycles per second,it

and the .channel W1 from east to west. The channel E3 is provided withamodulating apparatus M3 w ich may be of any well known type, such asfor example, a vacuum tube modulator. The modulator is supplied `with isarran ed to select the lower side band re- -Ez and `ficial lines ornetworks N3 sulting rom modulation. For this purpose,` the band filteris designed to have an upper l cut-olf frequency of 2,000 cycles. Itwill be understood, of course, that the line sections L1 and L3 will bebalanced in the usual manner by means of artificial lines or networks N3and N3. s

Similarly at station B a 22 circuit having channels1E3 and W3 isprovided for interconnecting the line sections L3 and L,3 and invertingthe band transmitted fromy east to west. The line sections L3 and L,3arebaL' anced with respect, to these channels by means of networks N3and N4 respectively, and the channel W3 is provided with a modulatingarrangement M3 and a band filter BF3 similar to the modulating apparatusand band filter in the channel El'at station A.

At an intermediate point C a 22 form of circuit or anization is arrangedto interconnect the line sections L3 and L3 for vthe purpose ofreinverting the inverted bands transmitted to this pointfrom theterminal stations so that the bands may be transmitted from this pointto'the terminal stations in. thenormal frequency relation. The circuitorganization 'at station C comprises channels connected as indicated,the artiand N3l being provided to balance the line sections L3 and L3.Modulating apparatus M3 and M3 is included inthejchannels E3 and W3respectively, this apparatus being similar tothe modulating apparatusalready` described. Band filters BF3 and BF3 are also included in theoutput circuits of the modulators, these band filters being similar tothe band filter at station A.

Further detailsfof the apparatus will be acquired from the description4of the 0peration, which is as follows: Assuming that a band of voicecurrents is to be transmitted fromthe station A tothe station B, theband of frequencies-willpass from the line section L1 into the channelE1 to modulate the carr1er frequency of 2,000 cycles, which coincideswith the upper limit of the voice band. In orderto understand theoperation more clearly the processes to which one frequency in the voicelrange is subjected in passing from station A to station B will beconsid# lower limit of condition,

ered. A frequency of 200 cycles, 'for example, in passing into.v themodulator M1 modulates the carrier frequency of 2,000 cycles to producefrequencies of 1,800 cycles andV 2,200 cycles. The-u per frequency iseliminated 'by theband lter BF1 and the lower frequency of 1,800 cyclespasses to the line section L3 and .over said line section into thechannel E3 at station C. It will be observed that during transmissionover the line section L2 the frequency relations are inverted and thefrequency of 200cycles which'normally lies near the lower side of thevoice band is translated to 1,800 cycles, which is correspondingly nearthe upper limit of the voice band. When the^1,800 cycle frequency passesintothe modulator M3 at station C it modulates the carrier frequency of2,000 cycles to producefrequencies of 200 cycles, and 3,800 cyclesrespectively.y The upper of these two frequencies is eliminatedby theband filter BF3 and the 200 cycle frequency,` which it will be noted isnowA restored to normal condition, is transmitted over the line sectionL3 and passes Jthrough the channel E3 to the line section L3, at stationB. It will be readily apparent that the voice 'band was inverted duringtransmission over the'line section L3 and was reinverted and transmittedover the line section L3 in normal so that on the average all frefquencies were subjected to substantially the same attenuation.

The vtransmission from station B to station A is similar `to thatalready described. A frequency of 200 cycles incoming from the linesection L3 passes through the modulator M3, is translated to a frequencyof 1,800 cycles and after transmission over the line section L3 istranslated by the modulator M3 tothe normal frequency of 200 cycles,which in turn is transmitted over the line section L3 and through thechannel W1 to the line sectin L3.

Figure 4 indicates schematically an arrangement for obtainingthe sameresult by means of a four-wire. circuit extending between station A andB. 'For' transmission from west4 to east, the line sections L1L and L7at stations A and B respectively are interf connected by a transmissionline com risverting apparatusis provided between each of the other linefrom west t-o east, and the circuit used 1n transmitting from east towest is also prosections used in transmitting vided with band invertingapparatus at the junction point of each line section.- 'The circuitasthus arranged will result in normal transmission over the linesections -marked Normal and inverted transmission over the line sectionsmarked linverted. If line sections 'L2 and L2 as Well as line sectionsL5 and L5', are made of onehalf the length of the other lline sections,the other line sections being made of equal length, it will be apparentthat the telephone band will be transmitted inverted over onehalf of thedistance between A and B and will be transmitted normal during the otherhalf. The same result may, of course, be obtained by making the linesections unequal, but in this case it is desirable that the totallengths of the sections transmitting the band inverted be substantiallyequal to the total lengths'of the sections transmitting the band innormal condition.

Assumin a band. incoming rom the line L1 is to. be transmitted tostation B, the band will be transmitted over the half line section L2where, by modulating with a carrier frequency of 2,000 cycles, and bysuppressing the upper side band, the band will be inverted fortransmission over the section L3. The in verted band, by'means of themodulator M1, and band filter BF2, will in tunn be reinverted andtransmitted in normal frequency relations over the section L4. It isagain inverted by the apparatus M4 and BF, and 'transmitted with thefrequencies in inverse relationover the section L5, after which it isrestored to normal' by the modulator M5 and the band filter BF5 andtransmitted in normal condition to the line L?, The trans- ,inission inthe opposite direction is similar, the band being transmitted in normalcondition over the line section L5', inverted and transmitted over theline section L5', rein- `vert'ed and restored to normal for transmissionover. the line L4', again inverted for v transmission overthe vlinesection L2', and

finally restoredto normal for transmission over the line section L2.

' A somewhat similar arrangement is illustrated in Fig. 3, this figurediffe-ring from that of Fig. 2, however, in that the carrier frequency,instead of being'supplied locally at each -modulating point, istransmitted along with the telephone band or inverted band, as thecasemay be; fqrexa1nple, for transmission fromwest toeast the carrierfrequency is supplied at the terminal station A from" a3' source Slandis transmitted with the normal band tothe modulator M2,4wh'ere thetelephone band modulates-with the carcrie'r frequency to invert the bandin'the samemanner as in the case ofl Fig. 2.? The` modulating apparatusmust in this instance be of a type which does not sulppress the carrier,whereas in the case of 1g. 2, the

f telephone frequenciesto west, the carrier will be supplied by the.

source S7 at station B and transmitted with the normal or inverted sideband as the case may be, just as described in connection with,transmission from west toeast.

It will be evident that the method above described involves a form ofcarrier system. For the sections in which the band is invertalternatingcurrent whose frequency coincides with the upper limit of the telephoneor signalling band, while in sections in which the band is transmittednormal, the carrier or reference frequency is a zero frequency. Ininverting the telephone band, it will be advantageous ordinaml to keepit ed the carrier. or reference frequency is an l vas low in frequencyas possi le, thereby causing the inverted band to occupy substantiallythe same position in the broad frequency range as does the normal band.It will be evident, however, that the inverted cband may be made tooccupy any position in the broad frequency range and that in fact boththe inverted and non-inverted ordinary carrier rv'nels as it is forchannels operated at ordi-v nary signalling frequency. Verylong carriercircuitsmay, however, warrant inverting the band for ualizationpurposes. A convenient way o inverting the carrier bands obtains in asystem employing opposite side bands referred to the carrier .frequencyfor transmission east and west respectively. For example,l if the commoncarrier is 10,000 cycles, and one channel operates onthe upper band upto 12,000 cycles, and the oppositely directed channeloperates von the.lower band down to 8,000 cycles, these bands may be readilyinterchanged at a repeater point 'by being modulated in a commonmodulator, if desired, with 10,000 cycles.

l It will be understood that in the previous description the variousfrequencies referred to', and the apparatus described, is for puroses ofillustration only, and the invention applicable tofotherfrequenc rangesand withlother forms of apparatus than those referred to in thedescription and illustrated in the drawing.

It will also be obvious that the general principles herein disclosed maybe embodied in many other organizations widely different from thoseillustrated without departing from the spirit of the invention asdefined in the following claims.

What is claimed is:

1. The method of equalizing the attenua manner lduring transmission overthe remainder of the system.

2., The method of equalizing the attenuation of a band of frequenciestransmitted over a transmission system which consists in transmittingthe band in normal frequency-7 relation over a portion of the system,and transmitting the band in an inverted frequencyv relation and in thesame frequency range over the remainder of the system.

Y 3. The method of equalizing the attenuation of a band of frequenciestransmitted over a transmission system which consists in transmittingthe band over a portion of the system in normal condition, modulatingthe band with a carrier frequency, thereby producing side bands,selecting the lower side band which correspondsto the original bandinverted,'fand trasmitting the inverted Aband over another fportion ofthe system.

4. The method o equaliing the attenuation of a band of frequenciestransmitted over a transmission system which consists in transmittingthe band over a portion of said system in normal-condition, modulatinthe band withl a carrier frequency in the neighobrhood of, the upperlimit of the which corresponds to the original band with frequenciesinverted and transmitting the inverted band. overc another portion1 ofthe system. f n f g 5. A system for (the equal attenuation of a-band oftransmitted frequencies comprising a system of'conductors over Which theband is transmittedl in normal frequency relation, means for invertingthe frequency relation of the band, and applying it to a similar systemof conductors so` that it will be transmitted over the second system ofconductors in inverted frequency relation.-

6. A system for the equal attenuation 'of a band of transmittedfrequencies comprising a system Aof conductors over which the band maybe transmitted in normal frequency relation, means for modulating theband in accordance with a carrier frequency,

thereby producing side` bands, means to select the lower side bandcorresponding to theloriginal band in inverted frequency relation, andto impresssaid inverted band upon a second similar system of conductors,whereby-the band may be transmitted over the second system of conductorsin inverted frequency relation.

7. A system for the equal attenuation of a band of transmittedfrequencies comprising a system of conductors over which a band offrequencies maybe transmitted in normal frequency relation, means formodulating said band of frequencies with a carricr whose frequencycorresponds substantially to the upper limit of the transmitted hand,thereby producing side bands, means for selecting the lower sidle bandcorresponding to the original band in inverted frequency order andimpressing said inverted band upon a second similari s stem ofconductors, so that the band may e transmitted. over said second systemof conducl tors in inverted frequency relation.

8. The method of compensating for the distortion in Wave shape producedby transmitting a band of signaling frequencies over a transmissionsystem, which consists in transmitting the signaling frequencies innormal frequency relation over a portion of the circuit to producedistortion of one character and transmitting said frequencies ininverted frequency relation and in the same frequency range over theremainder of the system to produce distortion of a complementarycharacter.

9. A system for compensating the distortion resulting from thetransmission of a band of signaling frequencies over a transmissionsystem, said system comprising conductors overA which the currents aretransmitted in normal frequency relation to produce distortion of onecharacter, and means for inverting the frequency relation of thesignaling currents and applying them to a.

similar system of conductors so that distortion of a complementarycharacter with 4l1aei" May, 1921. y

- vHERMAN A.

